[REF][PHP8.2] Avoid dynamic properties in mailstores

[braders]

Overview

Avoid dynamic properties in mailstores. See https://lab.civicrm.org/dev/core/-/issues/3833 for context.

Before

_mailbox, _ignored, _processed, _dir and _leftToProcess were declared dynamically, which will be deprecated in PHP 8.2.

After

The properties are explicitly declared.

Comments

I've declared the properties as private/protected. It's possible that something is trying to read one of the above properties in an extension, but I think it's unlikely - the mailer object is not passed into any hooks, which means any extension would struggle to get a refrerence to the mailer object in order to read it's properties.

That said, happy to change the properties to public if it would make people feel more comfortable with merging this.

[civibot]

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[eileenmcnaughton]

I'm not seeing universe references to MailStore other than @artfulrobot's patchwork - which seems to use this as an example so I think a ping is enough & we can go with the protected / private

[artfulrobot]

@eileenmcnaughton Thanks for the shout out, this change should not affect https://github.com/artfulrobot/mailstorepermissions (and, ironically, if it did then my extension could easily undo it anyway as it patches this file!)

[eileenmcnaughton]

@artfulrobot proof of concept!